dist/tools: script falsely accuses headers without C symbols of lacking `extern "C"` decl

[carl-tud]

dist/tools/externc/check.sh complains about headers not having an extern "C" declaration when there are no C symbols being defined in that header (e.g., the header only defines only macros).

[crasbe]

I'm not sure this is so easy to distinguish in a shell script. Do the extern "C" declarations do any harm in a header with only macros?